It is well known in the prior art to use an internal combustion engine as a brake means by converting the engine temporarily to an air compressor. The conversion starts by cutting off the provision of the fuel, opening the exhaust valve(s) at or near the end of the compression stroke of the engine piston, and allowing the compressed gases (air during braking) to be released. The energy absorbed by the compressed gas during the compression stroke of the engine can not be transmitted to the engine piston through the subsequent expansion stroke, but is dissipated by the exhaust and cooling systems of the engine, resulting in an effective engine braking. Thereby the vehicle is slowed down.
An example of the engine brake device is disclosed in U.S. Pat. No. 3,220,392 by Cummins, and an engine brake system based on the patent has achieved a great commercial success. However, this kind of engine brake system is a bolt-on accessory mounted at the top of the engine. In order to mount this kind of brake system, a spacer is additionally provided between the cylinder head and the valve cover, which adds unnecessary height, weight and costs to the engine. The above problems occur due to the fact that the engine brake system is employed as an accessory to, rather than an integrated part of, the engine.
The prior engine brake transmits the mechanical input to the exhaust valve(s) to be opened through a hydraulic circuit. A master piston reciprocating in a master piston bore is located in the hydraulic circuit. The reciprocating motion is provided by the mechanical input of the engine, such as the rocking of the injector rocker arm. The motion of the master piston is transmitted, through hydraulic fluid, to a slave piston located in the hydraulic circuit, causing the slave piston to reciprocate in a slave piston bore. The slave piston acts, directly or indirectly, on the exhaust valve(s), generating the valve event for the engine braking operation.
Therefore, the conventional hydraulic-driven engine brake has another drawback due to the compliance or deformable of the hydraulic system, which is relevant to the flexibility of the fluid. High flexibility of the fluid greatly reduces the brake valve lift. The reduction of the brake valve lift leads to the increase of the braking load, which in turn causes a higher flexibility, thereby forming a vicious circle. In addition, the brake valve lift reduction caused by the hydraulic deformation increases with the increase of the engine speed, which is against the engine braking performance requirement that higher engine speed needs higher brake valve lift. In order to reduce the hydraulic flexibility, a large diameter hydraulic piston is needed, which increases the volume and weight as well as the time of oil refill or discharge for extending or retracting such a large diameter piston. That is to say, a large diameter hydraulic piston will increase the momentum of inertia and response time of the engine brake system.